This invention relates to an improved process for delignifying and brightening lignocellulosic pulp. More particularly, the invention is directed at a peroxide-based delignifying and bleaching solution and process for lignocellulosic pulp. In this process, an aqueous peroxide solution of pH 1-7 is activated by the presence of certain metal containing additives whose metallic portions are selected from the group consisting of tin, titanium, and vanadium to delignify and brighten lignocellulosic pulp with minimal destruction of the cellulosic portion of the pulp. In a second aspect of this invention, these peroxide residuals remaining after tin activated delignification are consumed, following alkaline pH adjustment, in the further bleaching of the lignocellulosic pulp.
Wood is composed of two main parts--a fibrous carbohydrate or cellulosic portion and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin.
For use in paper-making processes, wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited as a screen to form a sheet. The methods employed to accomplish this pulping usually involve either a physical or chemical treatment of the wood or perhaps some combination of the two processes, to alter its chemical form to give desired paper properties.
In mechanical pulping, the wood is physically ground to a high-yield, lignin-retained pulp, most often referred to as groundwood pulp. In chemical pulping, the wood chips are digested with chemical solutions to solubilize a portion of the lignin and effect its removal. The more usual of these digestive procedure are the sulfite, sulfate or Kraft, soda and modified sulfite processes.
After the wood has been digested or mechanically ground, the resulting material is generally a darkly colored cellulosic fiber. The dark color is attributable to the fact that not all of the lignin has been removed during digestion and substantially none has been removed during mechanical processing. This dark pulp is commonly referred to as unbleached pulp. It may pass directly to the paper making operation if the paper color is unimportant.
More usually, the unbleached lignocellulosic pulps are bleached or brightened to a brightness consistent with the planned utilization of the pulp, brightness being a measure of pulp reflectivity under standardized conditions. Pulp bleaching is most often a multi-stage process employing various chemicals to remove or alter the lignin of the lignocellulosic pulp such that the resultant pulp is no longer light absorbing or dark in color.
Two classes of compounds generally find use as lignocellulosic brighteners, namely reducing agents and oxidizing agents. Common reducing agents include sulfurous acids, hydrosulphites, borohydrides, amine boranes, and bisulfites. Common oxidizing agents include chlorine-based compounds, peroxides, peracids, ozone, oxygen, permanganates and chromates.
The ready availability and environmentally appealing nature of hydrogen peroxide has made it increasingly popular in recent years in the paper-making industry. Such peroxide is used in the "lignin retaining" bleaching of groundwood and other high yield pulps to provide substantial brightness gains but not the high brightness attainable with chemical pulps. Such processes are described for example in Hook, "Peroxide Bleaching Opens New Potentials For Groundwood Pulps", Pulp & Paper International, 45-48 (June 1975), Vartiainen, "Utilization of Peroxide in Pulp Bleaching", Papper och Tra, 51, 277-284 (1969), U.S. Pat. Nos. 2,187,016, 3,023,140, and 3,251,731 and Canadian Pat. No. 970,111. Additionally, peroxides are employed as a "capping" or super-bleach stage at the end of common multi-stage bleaching processes for chemical pulps. Exemplifying these processes are those described in Canadian Pat. Nos. 966,604 and 970,111, U.S. Pat. Nos. 3,193,445 and 3,462,344 and Vartiainen, "Utilization of Peroxide in Pulp Bleaching", Papper och Tra, 51, 277-284 (1969), Delattre, "Hydrogen Peroxide as a Bleaching Agent for Kraft Pulps", Papper och Tra, 117-127 (1971), Hartler et al., "Peroxide Bleaching of Kraft Pulps", Tappi, 43, 806-813 (1960), Christensen, "Bleaching Sulphate Pulp With Hydrogen Peroxide", Pulp and Paper Magazine of Canada, 62-66 (1971), Christensen, "Bleaching of Sulphate Pulps With Hydrogen Peroxide", Norsk Skogindustri, 268-271 (October, 1973), and Mlakar & Peltonen, "Peroxide in the Semibleaching of Kraft Pulp", Papper och Tra, 11, 629-638 (1968).
However, such peroxide bleaching by itself is not able to effect sufficient pulp bleaching for most paper-making requirements. Previous variations of the conditions of such peroxide contact have either not remedied this inferior bleaching characteristic or have so degraded the cellulosic portion of the pulp that it was no longer useful for paper-making processes.
For example, at acid pH, brightness levels obtained are inferior to those from peroxide bleachings on the alkaline side. E.g., Rapson, "The Role of pH in Bleaching Pulp", Tappi, 39, 284-294 (1956). Moreover, absent special low temperature impregnation and drying procedures disclosed in U.S. Pat. No. 2,173,474, acid solutions of hydrogen peroxide effect marked deterioration of the pulp in both strength and viscosity. E.g., U.S. Pat. No. 2,112,116. Similar pulp viscosity reductions adduced by the addition of metals such as manganese, cobalt, iron, copper, and aluminum to oxidizing bleaching agents such as peroxide have been used to good advantage in U.S. Pat. Nos. 2,975,169 and 2,368,527 to produce pulp, which, while unsuitable for paper manufacture, is well constituted for the production of viscose rayon or other cellulose-like derivatives.
Other additives are likewise ineffective in improving the brightness gain possible with a single-staged hydrogen peroxide. For example, U.S. Pat. Nos. 2,119,519 and 2,249,646 disclose the addition of titanium having a valence of less than four to oxidizing bleaching agents, including peroxides. Such addition is said to activate the alkaline bleaching process so as to reduce both the treatment time and oxidant concentration needed to obtain a given level of brightness gain. These reductions advantageously benefit pulp strength since viscosity losses and other pulp degradation are minimized due to abbreviated oxidant-pulp contact.
The failure of hydrogen peroxide to bleach pulp in an internal or prebleaching stage to a substantial degree of brightness is apparently due to the inability of peroxide to delignify lignocellulosic pulps sufficiently to remove the light-absorbing, dark lignin components and yet, to maintain that resultant pulp viscosity and strength necessary for subsequent paper making processes.